Introduction:
In the treatment of water, various filter materials are used to remove impurities and contaminants. These filter materials can include sand, anthracite, crushed rock, and garnet sand. However, one of these options is not commonly used as a filter material in water treatment.

Explanation:
Sand:
- Sand is commonly used as a filter material in water treatment processes.
- It is effective in removing suspended solids, turbidity, and some microorganisms from water.
- The size and shape of sand particles allow for the formation of a porous bed that traps particles and allows for the passage of treated water.

Anthracite:
- Anthracite is another commonly used filter material in water treatment.
- It has a high carbon content and is effective in removing smaller particles, color, and odor from water.
- Anthracite filters can be used alone or in combination with other filter media, such as sand, for enhanced filtration.

Crushed rock:
- Crushed rock, or gravel, is not commonly used as a filter material in water treatment.
- It is primarily used as a support medium in filtration systems, providing stability and preventing the loss of finer filter media.
- Crushed rock does not have the same filtration properties as sand or anthracite and is therefore not suitable as a primary filter material.

Garnet sand:
- Garnet sand is also commonly used as a filter material in water treatment.
- It has high hardness and specific gravity, which allows for effective filtration of smaller particles.
- Garnet sand filters are often used in multi-media filtration systems, where different filter media are layered to achieve different levels of filtration.

Conclusion:
In summary, crushed rock is not commonly used as a filter material in the treatment of water. Sand, anthracite, and garnet sand are the more commonly used filter materials due to their filtration properties and effectiveness in removing impurities from water.